Operating Segments - Schedule of Information about Reportable Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Research services revenues	$ 52,727	$ 52,205	$ 160,998	$ 156,667
Advisory services and events revenues	24,700	22,548	81,651	76,084
Total revenues	77,427	74,753	242,649	232,751
Segment expenses	27,669	27,440	89,052	86,810
Contribution margin (loss)	49,758	47,313	153,597	145,941
Selling, marketing, administrative and other expenses	(41,998)	(40,820)	(130,208)	(126,120)
Amortization of intangible assets	(208)	(224)	(627)	(669)
Reorganization costs		(928)	(1,026)	(4,433)
Other income (expense) and gains (losses) on investments	(856)	404	(765)	578
Income before income taxes	6,696	5,745	20,971	15,297
Products Segment [Member]
Segment Reporting Information [Line Items]
Research services revenues	52,727	52,205	160,998	156,667
Advisory services and events revenues	2,333	2,041	14,191	12,763
Total revenues	55,060	54,246	175,189	169,430
Segment expenses	8,884	7,918	30,306	27,799
Contribution margin (loss)	46,176	46,328	144,883	141,631
Research Segment [Member]
Segment Reporting Information [Line Items]
Advisory services and events revenues	10,643	9,804	33,905	31,454
Total revenues	10,643	9,804	33,905	31,454
Segment expenses	12,548	12,711	38,707	38,467
Contribution margin (loss)	(1,905)	(2,907)	(4,802)	(7,013)
Project Consulting [Member]
Segment Reporting Information [Line Items]
Advisory services and events revenues	11,724	10,703	33,555	31,867
Total revenues	11,724	10,703	33,555	31,867
Segment expenses	6,237	6,811	20,039	20,544
Contribution margin (loss)	$ 5,487	$ 3,892	$ 13,516	$ 11,323